Solar
Radiation

We
need energy to support all our activities but most of our energy comes
now from fossil fuels. Do we have to use them?

For
a time being we have to use them because we do not yet have a convenient
replacement, but we have to work harder and faster to develop alternative
sources of energy; otherwise the damage we do to the environment is
likely to have catastrophic consequences.

One
of these alternative sources of energy comes from nuclear fusion reactions
in the Sun. The Sun delivers about 7000 times more energy to the Earth's
surface than we currently need for our global consumption.Even if we
could use only a small fraction of the solar energy, we could reduce
our dependence on fossil fuels. In time we might even eliminate them
altogether. We could clean the environment and improve the living conditions
on our planet.

NUCLEAR
FUSION REACTOR

The
Sun is a powerful nuclear fusion reactor producing staggering amounts
of energy, which unfortunately is dispersed in space and practically
all of it is lost. The Earth is 149,596,000 km from the Sun and at this
distance solar flux is relatively small.

The
energy intercepted by the Earth over a period of one year is equal to
the energy emitted by the Sun in just 14 milliseconds. To put it another
way, solar energy captured by the Earth over a period of 1000 years
is equal to the energy produced by the Sun in just only 14 seconds.

SOLAR
RADIATION REACHING THE EARTH

The
intensity of the solar radiation reaching us is about 1369 watts per
square metre [W/m2]. This is known as the Solar Constant.
It is important to understand that it is not the intensity per square
metre of the Earth’s surface but per square metre on a sphere
with the radius of 149,596.000 km and with the Sun at its centre.

The
total solar radiation intercepted by the Earth is the Solar Constant
multiplied by the cross section area of the Earth. If we now divide
the calculated number by the surface area of the Earth, we shall find
how much solar radiation is received, on average, by a square metre
of the Earth's surface. Thus, the average solar radiation S per square
metre of the Earth’s surface is,

where
S in the Solar Constant in W/m2 and r
is the Earth's radius.

SOLAR
RADIATION REACHING THE EARTH SURFACE

However,
our calculations are not yet finished because we have not yet considered
the influence of the Earth's atmosphere. The value we have calculated
is for the average solar radiation intensity at the outer regions of
the Earth’s atmosphere. What we want to know is how much of this
radiation reaches the earth surface where we are.

The
atmosphere absorbs about 68 W/m2 and reflects 77 W/m2
(Wallace and Hobbs 1977). The radiation reaching the Earth’s surface
is therefore on average 198 W/m2, i.e. 58% of the radiation intercepted
by the Earth.

Figure
1. The distribution of the solar radiation. On average,
each square metre of the upper regions of the atmosphere receives
342 watts of solar radiation [W/m2]. The atmosphere
absorbs on average 67 W/m2 and reflects 77 W/m2.
About 198 W/m2 reaches the Earth's surface, of which 168 W/m2
is absorbed and 30 W/m2 is reflected back to space.
The total of the reflected radiation is 107 W/m2,
or 31% of the incoming radiation.

Source:
Modified figure of Houghton et al. 2001

The
intensity of solar radiation depends on the time of the year and geographical
positions as illustrated in Figure 2.

Figure
2:
The intensity of solar radiation (solar power) in various parts
of the world depending on the season, measured in watts per
square metre [
W/m2].

Using
the Solar Constant we can calculate (see the formula below) that the
total solar energy intercepted by the Earth in one year is 5.5 million
exajoules [EJ/y]. To appreciate this figure we have to compare it with
the global energy consumption in 2005, which was only 463 EJ/y. Thus,
even though the Sun is so far from us, we still receive huge amounts
of energy from this immensely powerful source.

The
energy reaching the Earth surface is 3.2 million EJ/y, which is close
to 7000 times the global energy consumption. If we could harvest even
a small fraction of this energy we could solve our energy problems.
Global energy consumption in2005 was only 0.014% of the solar energy
reaching the Earth surface. The projected global consumption in 2100
is 0.051%. We should therefore expect that we should be able to harvest
enough of solar energy to replace the harmful fossil fuels.

HANDY
FORMULA

Here
is the formula you can use to calculate how much energy we receive from
the Sun:

where
E is the solar energy in EJ, S is the Solar Constant
in W/m2, n is the number of hours, r is
the Earth's radius in km.

This
formula is for the total solar energy intercepted by the Earth in n
hours. If you want to calculate how much of the solar energy reaches
the Earth's surface, multiply the result by 0.58.

You
can use this formula and Figure 1 to calculate how much solar energy
is reflected or absorbed by the Earth and the atmosphere. Using information
contained in The
Little Green Handbook you can also calculate that our current global
annual consumption of energy is equal to the average solar energy reaching
the Earth's surface over a period of only one hour and 16 minutes. Add
to it an extra 30 minutes and you'll have enough energy for the annual
global consumption in 2020.

How
much solar energy can we harvest? Can it be enough to satisfy our global
energy needs? See my estimations
in another section.

Copyright

You
may use the information contained in this article as long as you refer
to it as Nielsen, R. 2005, 'Solar Radiation', http://home.iprimus.com.au/nielsens/.